The invention relates in general to piezoelectric resonators and to filters having piezoelectric resonators. In particular, the invention relates to a resonator structure, which is quite simple to manufacture and has good electrical properties.
The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units. The development leads to increasing requirements on the miniaturization of the components and structures used in the mobile communication means. This development concerns radio frequency (RF) filter structures as well, which despite the increasing miniaturization should be able to withstand considerable power levels, have very steep passband edges, and low as losses.
The RF filters used in prior art mobile phones are often discrete surface acoustic wave (SAW) filters or ceramic filters. Bulk acoustic wave (BAW) resonators are not yet in widespread use, partly due to the reason that feasible ways of combining such resonators with other circuitry have not been presented. However, BAW resonators have some advantages as compared to SAW resonators. For example, BAW structures have a better tolerance of high power levels.
It is known to construct thin film bulk acoustic wave resonators on semiconductor wafers, such as silicon (Si) or gallium arsenide (GaAs) wafers. For example, in an article entitled xe2x80x9cAcoustic Bulk Wave Composite Resonatorsxe2x80x9d, Applied Physics Letters, Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is disclosed which comprises a thin film piezoelectric layers of zinc oxide (ZnO) sputtered over a thin membrane of silicon (Si). Further, in an article entitled xe2x80x9cAn Air-Gap Type Piezoelectric Composite Thin Film Resonatorxe2x80x9d, 15 Proc. 39th Annual Symp. Freq. Control, pp. 361-366, 1985, by Hiroaki Satoh, Yasuo Ebata, Hitoshi Suzuki, and Choji Narahara, bulk acoustic wave resonator having a bridge structure is disclosed.
FIG. 1 shows one example of a bulk acoustic wave resonator having a bridge structure. The structure comprises a membrane 130 deposited on a substrate 200. The resonator further comprises a bottom electrode 110 on the membrane, a piezoelectric layer 100, and a top electrode 120. A gap 210 is created between the membrane and the substrate by etching away some of the substrate from the top side. The gap serves as an acoustic isolator, essentially isolating the vibrating resonator structure from the substrate.
In the following, certain types of BAW resonators are described first.
Bulk acoustic wave resonators are typically fabricated on silicon (Si), gallium arsenide (GaAs), glass, or ceramic substrates. One further ceramic substrate type used is alumina. The BAW devices are typically manufactured using various thin film manufacturing techniques, such as for example sputter vacuum evaporation or chemical vapor deposition BAW devices utilize a piezoelectric thin film layer for generating the acoustic bulk waves. The resonance frequencies of typical BAW devices range from 0.5 GHz to 5 GHz, depending on the size and materials of the device. BAW resonators exhibit the typical series and parallel resonances of crystal resonators. The resonance frequencies are determined mainly by the material of the resonator and the dimensions of the layers of the resonator.
A typical BAW resonator consists of three basic elements:
an acoustically active piezoelectric layer,
electrodes on opposite sides of the piezoelectric layer, and
acoustical isolation from the substrate.
The piezoelectric layer may be for example, ZnO, AlN, ZnS or any other piezoelectric material that can be fabricated as a thin films as a further example, also ferroelectric ceramics can be used as the piezoelectric material. For example, PbTiO3 and Pb(ZxTi1-x)O3 and other members of the so called lead lanthanum zirconate titanate family can be used.
The material used to form the electrode layers is an electrically conductive material. The electrodes may be comprised of for example any suitable metal, such as tungsten (W), aluminum (Al), copper (Cu), molybdenum (Mo), nickel (Ni), titanium (Ti), niobium (Nb), silver (Ag), gold (Au), and tantalum (Ta). The substrate is typically composed of for example Si, SiO2, GaAs, glass, or ceramic materials.
The acoustical isolation can be produced with for example the following techniques:
with a substrate via-hole,
with a micromechanical bridge structure, or
with an acoustic mirror structure.
In the via-hole and bridge structures, the acoustically reflecting surfaces are the air interfaces below and above the devices. The bridge structure is typically manufactured using a sacrificial layer, which is etched away to produce a free-standing structure. Use of a sacrificial layer makes it possible to use a wide variety of substrate materials, since the substrate does not need to be modified very much, as in the via-hole structure. A bridge structure can also be produced using an etch pit structure, in which case a pit has to be etched in the substrate or the material layer below the BAW resonator in order to produce the free standing bridge structure.
FIG. 2 illustrates one example of various ways of producing a bridge structure. Before the deposition of other layers of the BAW structure, a sacrificial layer 135 is deposited and patterned first. The rest of the BAW structure is deposited and patterned partly on top of the sacrificial layer 135. After the rest of the BAW structure is completed, the sacrificial layer 135 is etched away. FIG. 3 shows also the substrate 200, a membrane layer 130, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. The sacrificial layer can be realized using for example ceramic, metallic or polymeric material.
In the via-hole structure, the resonator is acoustically isolated from the substrate by etching away the substrate from under a major portion of the BAW resonator structure. FIG. 3 shows a via-hole structure of a BAW resonator. FIG. 4 shows the substrate 200, a membrane layer 130, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. A via-hole 211 has been etched through the whole substrate. Due to the etching required, via-hole structures are commonly realized only with Si or GaAs substrates.
A further way to isolate a BAW resonator from the substrate is by using an acoustical mirror structure. The acoustical mirror structure performs the isolation by reflecting the acoustic wave back to the resonator structure. An acoustical mirror typically comprises several layers having a thickness of one quarter wavelength at the center frequency, alternating layers having differing acoustical impedances. The number of layers in an acoustic mirror is typically ranging from three to nine. The ratio of acoustic impedance of two consecutive layers should be large in order to present as low acoustic impedance as possible to the BAW resonator, instead of the relatively high impedance of the substrate material. In the case of a piezoelectric layer that is one quarter of the wavelength thick, the mirror layers are chosen so that as high acoustic impedance as possible is presented to the resonator. This is disclosed in U.S. Pat. No. 5,373,268. The material of the high impedance layers can be for example gold (Au), molybdenum (Mo), or tungsten (W), and the material of the low impedance layers can be for example silicon (Si), polysilicon (poly-Si), silicon dioxide (SiO2), aluminum (Al), or a polymer. Since in structures utilizing an acoustical mirror structure, the resonator is isolated from the substrate and the substrate is not modified very much, a wide variety of materials can be used as a substrate. The polymer layer may be comprised of any polymer material having a low loss characteristic and a low acoustic impedance. Preferably, the polymer material is such that it can withstand temperatures of at least 350xc2x0 C., since relatively high temperatures may be achieved during deposition of other layers of the acoustical mirror structure and other structures. The polymer layer may be comprised of, by example, polyimide, cyclotene, a carbon-based material, a silicon-based material or any other suitable material.
FIG. 4 shows an example of a BAW resonator on top of an acoustical mirror structure. FIG. 5 shows the substrate 200, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. The acoustical mirror structure 150 comprises in this example three layers 150a, 150b. Two of the layers 150a are formed of a first material, and the third layer 150b in between the two layers is formed from a second material. The first and second materials have different acoustical impedances as described previously. The order of the materials can be varied. For example, the material with a high acoustical impedance can be in the middle and the material with a low acoustical impedance on both sides of the middle material, or vice versa. The bottom electrode may also be used as one layer of the acoustical mirror.
FIG. 5 shows a further example of a BAW resonator structure. The BAW resonator illustrated in FIG. 5 is a stacked resonator structure having two piezoelectric layers 100. In addition to the bottom 110 and top 120 electrodes, a stacked structure requires a middle electrode 115, which is connected to ground potential. FIG. 6 further shows the membrane layer 130, the substrate 200 and the etch pit 210 isolating the structure from the substrate.
The cut-off frequency for a resonator is determined by assuming that the crystal resonator is infinite in the lateral direction. It is thus determined directly by the material of the layers in the resonator structure and by the thickness of the layers. The cut-off frequency is the mechanical resonance frequency of a laterally infinite plate.
The lateral dimensions of the resonator (or any plate) cause lateral resonance modes to emerge, and the basic resonance frequency of a resonator or that of a finite plate is somewhat higher or lower than its cut-off frequency. This fundamental later resonance mode or, in other words, the first mode lateral resonance corresponds to a situation, where there is an amplitude maximum in the middle of the resonator area.
In a finite plate there can be various mechanical vibrations, and any lateral resonance modes can be excited mechanical. Certain lateral resonance modes may be excited piezoelectrically, when an alternating voltage is exerted over the crystal. These lateral resonance modes that are usually at different frequencies cause the surface of the resonator to oscillate. The piezoelectrically excited strongest resonance mode is called the main mode and the other piezoelectrically excited modes are called spurious resonance modes. The spurious resonance modes usually occur at somewhat lower and/or higher frequencies than the cut-off frequency of a resonator.
One of the desired properties of a filter is that at the frequencies which the filter passes, the response of the filter is as even as possible. The variations in the frequency response are called the ripple. The frequency response of a filter should thus be constant, for example in a bandpass filter, over the bandwidth of the filter. In the blocking frequencies the ripple is usually not a problem.
The problem with the spurious resonance modes of crystal resonators and, for example, BAW resonators is that the ripple in filters that are constructed using these resonators is at least partly caused by spurious resonance modes of the resonators. This is discussed, for example, in an article entitled xe2x80x9cThin film bulk acoustic wave filters for GPSxe2x80x9d, in 1992 Ultrasonic Symposium, pp. 471-476, by K. M. Lakin, G. R. Kline and K. T. McCarron. The spurious resonance modes deteriorate the properties of systems that comprise crystal resonators or BAW resonators. The ripple in a frequency response of a filter is one example of the effect of the spurious resonances.
One of the goals of resonator design is to produce a resonator where the piezoelectrically excited strongest mode is a piston mode, where the amplitude distribution is flat across most of the resonator area. Usually, a resonator operating in the piston mode does not have strong spurious resonances. One of the main problems in resonator design is that, in general, the way how to make resonators operate in the piston mode is not known.
An object of the invention is to provide a resonator structure. A further object is to provide a resonator structure having good electrical response. A further object is to provide a resonator structure, where the displacement relating to the piezoelectrically excited strongest resonance mode is substantially uniform in an area covering a large part of the resonator; preferably the resonator structure operates in the piston mode. A further object of the invention is to provide a resonator structure that is easy to manufacture.
Objects of the invention are achieved by confining a center area of a resonator with a frame-like boundary zone, which has a different cut-off frequency than the center area, and by adjusting the properties of piezoelectrically excited resonance modes in the center area by selecting the acoustical properties and width of the frame-like boundary zone properly.
A resonator structure according to the invention is a resonator structure, where a certain wave mode is piezoelectrically excitable and which resonator structure comprises at least two conductor layers and at least one piezoelectric layer in between the conductor layers, said conductor layers and piezoelectric layer extending over a first area of the resonator structure, which first area is a piezoelectrically excitable area of the resonator structure, and which is characterized in that
the resonator structure comprises a frame-like zone confining a center area,
the center area is within the first area of the resonator structure,
a cut-off frequency of the piezoelectrically excited wave mode in the layer structure of the frame-like zone is different from the cut-off frequency of the piezoelectrically excited wave mode in the layer structure of the center area, and
width of the frame-like zone and acoustical properties of the layer structure in the frame-like zone are arranged so that displacement relating to the piezoelectrically excited strongest resonance mode is substantially uniform in the center area of the resonator.
An electrically excitable area of a resonator refers here to the area to which all the electrode layers and the piezoelectric layer(s) of the resonator extend. Usually the electrically excitable area is in the center of a resonator. In a resonator structure according to the invention, there is a frame-like zone that encircles a certain part of the electrically excitable area of the resonator. Term center area refers here to this part of the electrically excitable area, which is inside the frame-like zone. The center area does not have to be, for example, in the center of the resonator area.
The frame-like zone in a resonator according to the invention differs from the center area and from the area surrounding the frame-like zone in its acoustical properties. The cut-off frequency in the frame-like zone and/or the dispersion relation of the piezoelectrically excited wave mode in the frame-like zone may be different from those in the center area and/or in the area surrounding the frame-like zone. The cut-off frequency of a layer structure is determined by the thickness and acoustical properties of the layers, and by assuming that a plate having said layer structure is infinite. The dispersion relation depends on the material of the plates and on the acoustical wave mode (thickness extensional or shear), which is piezoelectrically excited in the resonator. The resonators according to the invention may operate in the thickness extensional mode or in the shear mode of fundamental (TE1, TS1) or higher order.
The acoustical properties and width of the frame-like zone in a resonator according to the invention are chosen so that when the resonator is excited piezoelectrically, the displacement of the strongest piezoelectrically excited wave mode is substantially uniform in the center area of the resonator. Consider a piezoelectrical plate having a certain thickness in vertical direction and electrodes on the horizontal surfaces. In the presence of a piezoelectrically excited thickness extensional wave the particles of the piezoelectrical material experience displacement in the vertical direction, in other words in the direction of the applied electrical field. In the presence of a piezoelectrically excited shear wave the particles of the piezoelectrical material experience displacement in the horizontal direction, in other words in a direction perpendicular to the applied electric field. When a resonance structure according to the invention is piezoelectrically excited, in the center area of the resonator there is a substantially uniform displacement. When the piezoelectrically excited wave is a thickness extensional wave, this means that the thickness of the center area varies as a function of time so that at each time instance the thickness of the center area, at substantially each point of the area, is the same. Similarly, when the piezoelectrically excited wave is a shear wave, the displacement of the particles is uniform in the horizontal direction. As an example of a uniform displacement, consider piston mode, where the displacement is uniform in a certain area of a resonator. In a resonator according to the invention, the uniform displacement related to the piston mode takes place in the center area of the resonator. Advantageously the center area operates in piston mode.
The active area of a resonator is the area where the acoustic wave has a considerable magnitude. It is possible that in a resonator according to the invention the center area covers most of the active area of the resonator, and consequently the electrical response of the resonator is dominated by the strongest piezoelectrically excited wave mode, advantageously by piston mode operation. The main advantage of the invention is thus that a resonator according to the invention exhibits good electrical response.
A suitable width and thickness for the frame-like zone can be estimated using a laterally one-dimensional model, as described below. It is also possible to find the optimum width and thickness for the frame-like zone experimentally.
The shape of the electrically excitable area of a resonator or the shape of the center area is not restricted to any particular shape in a resonator structure according to the invention. The center area in a resonator according to the invention may, for example, be rectangular, polygonal or circular. The width and acoustical properties of the frame-like zone are advantageously substantially uniform throughout the frame-like zone, but the resonator structures according to the invention are not restricted to such structures comprising a frame-like zone with uniform layer structure or with uniform thickness.
The center area of the resonator according to the invention is advantageously substantially uniform, in order to achieve piston mode operation. The thickness of the center area may vary slightly between the midpoint and the edges. In this case, piston mode operation is necessarily not achieved but the electrical response is still clean, in other words there are practically no spurious resonance modes.
The resonator structure according to the invention enhances the properties of conventional crystal resonators and especially the properties of thin-film BAW resonators. The properties of the prior-art BAW resonator types can be enhanced by modifying the structures according to the invention A resonator according to the invention may have, for example, a stacked structure.
In a resonator structure which has a frame-like zone whose width and thickness are selected properly, the strongest piezoelectrically excited mode in the center area of the resonator structure is piston mode. In such a structure, the spurious resonances occurring at frequencies near the piston operation frequency have often only a weak coupling, as discussed below in connection with a laterally one-dimensional model. This effect typically enhances the electrical properties of a resonator according to the invention even further.
When the properties of the resonators are enhanced, the properties of the components that comprise resonators are improved. Specifically, it is advantageous to manufacture filter using the resonator structures according to the invention. Such filters may be used, for example, in mobile communication devices.
Typically when a frame-like zone of a resonator is designed so that the strongest piezoelectrically excited mode in the center area of the resonator structure is piston mode, the resonator can be operated at a relatively wide frequency range around the piston mode operation point, because the anharmonic spurious modes are suppressed. A resonator can be designed to operate somewhat below or above the piston mode frequency to obtain an optimum overall response for a particular purpose. For example in a bandpass filter the ripple in the pass band may be minimized.
A further advantage of the invention is that the manufacture of resonators according to the invention does not necessarily require any additional manufacture steps. This is discussed in more detail in connection with the preferred embodiments of the invention.
The invention relates also to a filter comprising at least one resonator structure, where a certain wave mode is piezoelectrically excitable and which resonator structure comprises at least two conductor layers and at least one piezoelectric layer in between the conductor layers, said conductor layers and piezoelectric layer extending over a first area of the resonator structure, which first area is a piezoelectrically excitable area of the resonator structure, and which is characterized in that
the resonator structure comprises a frame-like zone confining a center area,
the center area is within the first area of the resonator structure,
a cut-off frequency of the piezoelectrically excited wave mode in the layer structure of the frame-like zone is different from the cut-off frequency of the piezoelectrically excited wave mode in the layer structure of the center area, and
width of the frame-like zone and acoustical properties of the layer structure in the frame-like zone are arranged so that displacement relating to the piezoelectrically excited strongest resonance mode is substantially uniform in the center area of the resonator.